122 STRUCTURE OF THE EYE. 



passes in front into the glassy, transparent cornea,//, 

 h h ; the greater part of the centre of the eye is occupied 

 by a clear gelatinous mass, the vitreous humor, G, in 

 front of which is the lens, L ; while between the lens and 

 the cornea is the aqueous humor, W. The sclerotic 

 coat is lined at the back of the eye by a delicate, 

 vascular, and pigmented membrane — the choroid, p-^, so 

 called from the great number of blood-vessels which it 

 contains ; in front this membrane joins the iris, p p, 

 which leaves a central opening, the pupil, so called 

 from the little image of ourselves, which we see re- 

 flected from an eye when we look into it. The iris gives 

 its colour to the eye, its posterior membrane con- 

 taining pigment-cells ; if these are few in number, it 

 appears blue, from the layer behind shining through, 

 and the greater the number of these cells the deeper 

 the colour, e e, is a peculiar membrane, which serves to 

 retain the lens in its place. The optic nerve, d, enters 

 at the back of the eye, and, spreading out on all sides, 

 forms the retina, i, of which one spot, s, the yellow spot, 

 is pre-eminently sensitive. The action of the eye re- 

 sembles that of a camera obscura, and, as shown in 

 Fig. 76, the rays which fall upon it are refracted so 

 as to form a reversed picture on the back of the eye. 



The retina (Fig. 78) is very complicated, and, 

 though no thicker than a sheet of thin paper, consists 

 of no less than nine separate layers, the innermost 

 (Figs. 78, 79) being the rods and cones, which are the 

 immediate recipients of the undulations of light. 

 Fig. 79 represents the rods and cones isolated and 

 somewhat more enlarged. 



The number of rods and cones in the human eye is 

 enormous. At a moderate computation the cones may 



